1. Field of the Invention
The present invention relates to image display devices.
2. Description of the Prior Art
Recently, in television sets used in general homes, a flat-panel display such as a liquid-crystal display device or a plasma display device has become popular instead of a Braun-tube display. The technology of a high-luminance flat-panel display has progressed with the improvement of device performance. However, although the technology of a high-luminance display device improves the visibility thereof, the technology has also led an important cause of tiredness or eye-fatigue due to glare feeling.
Moreover, a problem has also occurred that the cost of electric-power consumption increases with the high luminance of the liquid-crystal display device or the plasma display device.
Accordingly, a method is generally used in which, by installing into a display device a light receiving element for measuring the brightness level of the environment thereof, the display luminance of its screen is controlled with the environmental brightness. When the environment is relatively dark, the display luminance is set lower, while when the environment is relatively light, the display luminance is set higher. Thus, the visibility can be secured in the light environment, the glare feeling can be prevented, and the electric-power consumption can also be reduced.
For example, in a conventional display device, based on the peripheral light of the display screen, the screen luminance is automatically controlled to a brightness level at which an observer can comfortably see the screen. Specifically, the light receiving element is placed, towards the observer, close to the display screen. The brightness level dose to the display screen is measured by this light receiving element. The range of the display-screen luminance is calculated based on this measurement result, and the brightness level of the display device is controlled. The luminance of the display screen is calculated from the brightness level dose to the display screen and a constant number (Japanese Laid-Open Patent Publication No. 1994-308891).
Moreover, in another conventional display device, the brightness is adjusted to personal preference. In this control method, any one of plural functions is selected, which have been preset to calculate the luminance where the environmental illuminance is set as a parameter. The luminance corresponding to the environmental illuminance is obtained based on the selected function. Back light is lightened with the obtained luminance. The environmental brightness is measured using a brightness sensor similarly to the method disclosed in Patent Document 1, and by selecting a predetermined luminance-control setting value, comfortable environment that is matched to the personal preference is set (Japanese Laid-Open Patent Publication No. 2002-323690).